The use of pilot valves for sensing flow line pressure variations and for controlling the flow of a pressurized fluid in response to those variations is well known. Pilot valves typically comprise a cylindrical body having a centrally disposed longitudinal bore. One end of the valve contains a pressure sensor that is acted upon by the pressurized fluid in the flow line on which the pilot valve is installed. The other end of the valve is closed and contains a spring adapted to exert a force in opposition to that exerted on the pressure sensor by the pressurized fluid. A plunger inside the valve slides back and forth within the bore in response to variations in flow line pressure. The position of the plunger within the longitudinal bore at a given time is determined by the magnitude of the flow line pressure relative to the pressure exerted by the spring.
The body of a typical convertible high or low pressure pilot valve also contains three longitudinally spaced, radially directed ports that communicate with the valve bore. These ports are commonly referred to as the supply port, exhaust port and common port. The particular port that is employed as either the supply port or the exhaust port will depend on whether the valve is installed as a high or low pressure pilot. The plunger is adapted to selectively permit fluid communication between the common port and the supply or exhaust port as it moves back and forth within the bore in response to variations in flow line pressure. O-ring seals are typically employed between adjacent ports to restrict fluid flow in the annular space between the plunger and the longitudinal bore.
Convertible high or low pressure pilot valves are usually installed in series in a control loop that also contains a relay valve. When either the high or low pressure sensing pilot valve trips, the relay valve loses pressure and actuates a surface safety valve, shutting-in the flow line.
Pilot valves employing conventional designs are disclosed in the following U.S. Pat. Nos. 3,043,331; 3,621,881; 3,746,047; 3,888,457; 4,084,613; 4,209,040; and 4,217,934. In some cases, fluid communication between selected ports is achieved by varying the diameter of the plunger, thereby creating spool-shaped segments of sufficient length to provide communication between adjacent ports whenever the plunger is appropriately positioned. In other cases, the plunger is adapted by means of interior passageways to provide fluid communication between ports as desired whenever the plunger is in the appropriate position. Although many different models of pressure sensing pilot valves are commercially available, certain problems and difficulties have been experienced in the manufacture and use of these valves that have impaired their desirability, usefulness and reliability, and have highlighted the need for the innovative improvements disclosed herein.
One problem experienced with conventional pressure sensing pilot valves relates to the O-ring seals that are employed between adjacent ports to restrict fluid flow in the annular space between the longitudinal bore and the plunger. These seals are typically made of rubber, and are much more subject to wear and degradation than other parts of the valve. This is particularly true where the valve is designed and constructed in such manner that the seals are seated on the slide or plunger and are required to pass over ports or orifices in the valve body during use. Problems are especially apparent where the ports or orifices are relatively large compared to the size of the O-rings and where the edges of the ports or orifices are sharp or rough.
Problems have also arisen with pilot valves in which the O-rings are subjected to large pressure differentials as they are moved past parts or orifices. In such cases the O-rings are extruded into the open spaces, resulting in undesirable bypass or leakage.
A convertible high or low pressure pilot valve is therefore needed in which the O-ring seals are not required to pass over large diameter ports or orifices and are not subjected to undesirable pressure differentials when moving from one flow configuration to another.